1. Field of the Invention
The present invention relates generally to the field of semiconductor device fabrication and more particularly to apparatus and methods for planarizing surfaces on wafers.
2. Description of the Prior Art
In the field of semiconductor device fabrication, as well as in related manufacturing technologies, the ability to achieve greater miniaturization of devices such as integrated circuits (ICs) relies, in part, on the ability to create ever smoother and flatter surfaces. Planarization is the process of making surfaces more planar, and is commonly performed on wafers at multiple stages of semiconductor fabrication. One technique that is employed to planarize metal, insulator, and semiconducting layers is chemical mechanical polishing (CMP). As the name implies, CMP employs a combination of mechanical polishing and chemical reactions to remove surface materials to produce very flat and smooth surfaces.
Linear CMP is a particular CMP technique that employs a moving CMP belt to polish a wafer. The CMP belt also transports a slurry of reactants to the wafer and removes both reaction products and heat. In order to facilitate the transport of slurry to the wafer, and of reaction products away from the wafer, the CMP belt preferably includes a plurality of grooves. Often, to direct slurry towards the wafer, the grooves are oriented longitudinally, in the direction of travel of the CMP belt. The orientation of the grooves can also include a lateral component. The slurry is typically dispensed onto the CMP belt from a single nozzle or, commonly, from a slurry bar having multiple nozzles (typically 6–15). Ideally, the depth of the slurry as measured across a width of the CMP belt evens out before the slurry reaches the wafer, however, in practice the slurry frequently is unevenly distributed, forming a longitudinal band on the CMP belt for each of the nozzles of the slurry bar.
In linear CMP systems the wafer is typically secured in a polish head. During polishing, the polish head presses the wafer down against the CMP belt while rotating the wafer about a vertical axis. In one type of linear polishing system, air delivered through holes in a platen situated on the opposite side of the CMP belt from the wafer supports the moving CMP belt against the pressure from the polish head. It will be appreciated that although rotating the wafer tends to even out the effects of unevenly distributed slurry and of the grooves in the CMP belt, this smoothing effect is imperfect. Additionally, the benefits of wafer rotation lessen considerably at both the center and at the circumference of the wafer.
For example, as the polish head presses the wafer against the CMP belt, the polish head defines a wafer track on the CMP belt and forces slurry laterally out from the wafer track. This slurry tends to accumulate in two parallel bands on the CMP belt near the two edges of the wafer track. Accordingly, although the points on the wafer near the circumference traverse the greatest width of the CMP belt with each revolution, points near the circumference of the wafer also pass through these bands of excess slurry twice per revolution and tend to become over-polished. Similarly, the longitudinal bands of slurry that correspond to the nozzles of the spray bar can also create rings of over-polishing on the wafer, though this effect is typically less extreme than at the circumference.
A similar effect can occur at the center of the wafer. Specifically, it will be appreciated that although the wafer is rotating, a point at the center of the wafer is polished only by a single longitudinal line at the center of the wafer track, and, similarly, points on the wafer very near to the center are polished only by a narrow band of the CMP belt that is centered on that center line of the wafer track. If this narrow band includes one of the longitudinal grooves, or happens to correspond to a region with an excess or a deficient amount of slurry, then the center of the wafer will be polished differently than the remainder of the wafer and can become either over-polished or under-polished. FIG. 1 shows profiles across several wafers to illustrate the problems with polishing uniformity as a function of wafer radius.
Another problem with linear CMP systems is that the CMP belts wear unevenly. With the use of certain slurry types and belt groove patterns, the wafer track on the CMP belt can become smoothed, commonly referred to as glazing, which causes loss of removal performance. Since every point on the wafer crosses the center line twice per revolution, the center of the wafer track performs the most polishing, and therefore the center of the wafer track can glaze faster than the remainder of the wafer track. This uneven glazing of the CMP belt within the wafer track causes under-polishing towards the center of the wafer.
Therefore, what is needed is an apparatus and method to promote greater polishing uniformity in linear CMP systems.